In gathering data from a large number of sensors, two general types of methods have been used. In the first, a wire pair is run from each sensor to a data recording unit. In the second, some form of multiplexing is used so that data from many sensors is impressed on a data bus consisting of a single wire pair, coaxial cable, or optical cable. In practicing the second type of method, a saving in wire (or other data transmission material) and space for cable runs is realized. However, in practicing conventional embodiments of such type of method, a significant amount of electronic equipment has generally been required to digitize and encode information from each sensor input location. In practicing the method of the present invention, the advantages of multiplexing are obtained, and the amount of electronic equipment required at each sensor-data bus interface is reduced.
One important application for the present invention is in the field of marine seismology. In marine seismology the most commonly employed technique for obtaining geophysical data is the reflection seismograph technique which typically requires the use of a large number of hydrophone arrays connected to form what is known as a "marine streamer." The marine streamer is towed behind a seismic vessel. The individual hydrophones may be made up of a piezoelectric element which converts acoustic signals to electrical signals. Marine streamers typically use electrical cables to transmit such electrical signals from the submerged hydrophones to instruments which display or record these signals on board the seismic vessel.
A typical marine streamer may have 200 hydrophone arrays. Each array may be 15 meters long and may be made up of 17 hydrophones in parallel. Such a marine streamer would be three kilometers long, would have 3400 hydrophones, and would require at least 400 wires running the length of the electrical cable to connect each array with the vessel. In addition, other wires would be needed for depth measurement, control, and other purposes. The cable diameter necessary for accommodating such a large number of wires would be about 3 inches.
Longer marine streamers are desirable, but extension of the apparatus commonly used in the art would be awkward because of the need for increased cable diameter to accommodate such increased length. Another approach that has been taken utilizes a digital streamer. In this type of system, the data from each array is digitized, multiplexed, and then transmitted down a data bus to instruments on board the seismic vessel. This digital streamer approach, although allowing smaller diameter streamers, results in a more expensive system in the water, and usually requires relatively large diameter electronics packages positioned at various locations along the streamer which act as noise sources as the streamer is dragged through the water.
Systems have been proposed which employ optical transducers for converting acoustic vibrations incident on a device such as a hydrophone or geophone into optical signals, and then into electrical signals. Such systems would replace the conventional piezoelectric transducers with generally more complex fiber optic transducers. The problem of transmitting many such signals down the streamer remains the same.
One method of alleviating the problem of increased cable diameter is through the use of optical fibers in place of the electrical wiring. Fiber optic systems have been proposed which convert incident acoustic vibrations into optical signals and maintain such optical signals in optical form for transmission. Some of such previously proposed systems require a separate fiber (or fiber pair) for each sensor. Others of such previously proposed systems employ couplers and lossy sensors which cause an excess optical loss each time light propagates through them, and hence severely limit the number of signals which practically can be handled.
One method for producing a reflection in an optical fiber is described in U.S. Pat. No. 4,545,253 issued Oct. 8, 1985 to Avicola. This method employs evanescent coupling between two fiber segments separated by a looped section of fiber to cause a portion of light propagating down the fiber to be coupled from one segment to the other segment so as to propagate back along the fiber in the opposite direction. The fiber can be unbroken in this method. Although the excess losses in this arrangement are significantly lower than with a coupler, they are still too large to permit hundreds of such reflection points on a single fiber. Furthermore, the reflectors of U.S. Pat. No. 4,545,253 are permanent in the sense that the excess loss results every time light traverses the reflectors and such losses affect operation of all the reflectors formed downstream from any particular reflector on a single fiber.
Other known methods for producing a reflection in an optical fiber include: introducing a discontinuity in the fiber such as by breaking the fiber and reconnecting the broken ends; mechanically introducing a microscopic taper to the fiber; and exposing a portion of the fiber to spatially periodic pertubations of the optical refractive index of the cladding surrounding the fiber core. The latter method is described in U.K. Pat. Application No. GB 2,145,237A by Chevron Research Company, published Mar. 20, 1985, at page 5, lines 65 through page 6, line 35. These known methods of forming a reflector on a fiber all have the disadvantage that they result in a permanent reflector. There is an optical loss at each permanent reflector whenever light passes through it, and such losses affect all sensors associated with all the reflectors downstream of any particular reflector on a fiber. As a result of the losses, a large number of such permanent reflectors could not be accommodated on a single fiber.
Another type of fiber optic transducer mechanism relies on phase modulation in a single mode fiber immersed in a fluid. The phase modulation in such a system is due to changes in the optical length of the fiber induced by sound waves propagating in the fluid. An example of such technique is described in J. A. Bucaro, H. D. Dardy, and E. F. Carone, "Fiber-optic hydrophone", Journal Acoustic Society of America, Vol. 62, No. 5, pp. 1302-1304, 1977. The Bucaro paper does not teach or suggest any system in which several sensors are formed on the same fiber, nor does it teach or suggest any sensor that reflects a portion of an interrogating light signal for subsequent detection and processing.
A related optical transducer system is disclosed in U.S. Pat. No. 4,313,185 issued Jan. 26, 1982 to Chovan. Chovan discloses a hydrophone system comprising a first and a second single mode optical fiber and means for coupling light from the first fiber to the second fiber and from the second fiber to the first fiber. The optical length of the optical coupling path between the two fibers is modulated in response to acoustic vibrations incident on the fibers. The phase and frequency of light traversing the optical coupling path will vary with the optical length of the path and the rate of change thereof, respectively. Chovan, however, neither teaches nor suggests any system in which several sensors are formed on the same fiber, nor does it teach or suggest any sensor that reflects a portion of in interrogating light signal for subsequent detection and processing.
Other typical sensors and multiplexing schemes are described in the paper by E. L. Green, et al. entitled "Remote Passive Phase Sensor," presented at the Third International Conference on Optical Fiber Sensors held in San Diego, Feb. 13-14, 1985; and in above-referenced U.K. Pat. Application No. GB 2,145,237A. The system of the U.K. Patent Application includes a number of sensors formed on an optical fiber which are interrogated by an interrogating wavelength scanning laser signal. Each sensor includes a pair of reflectors designed so as to reflect a particular wavelength band of the swept frequency interrogating signal. The reflected signals are detected and processed in accordance with complex, wavelength division demultiplexing technique. The complexity of the demultiplexing technique is an important disadvantage. Furthermore, the methods for forming the reflectors disclosed in the U.K. Patent Application all have the disadvantage that they result in a permanent reflector, so that there is an optical loss at each permanent reflector whenever light passes through it. Such losses will affect all sensors associated with all the reflectors formed downstream of any particular reflector on the fiber.
The Green, et al. paper describes another technique for interrogating a remote interferometric sensor formed on an optical fiber by analyzing reflected light produced in the sensor when an interrogating light signal passes through the sensor. The sensor of the Green, et al. system includes a partial reflector and a full reflector. The system also includes a compensating interferometer defining two paths of different length. The path difference compensates for the time delay between the back reflected beams from the two reflectors. After propagating through the two paths, the reflected light is recombined and the phase of the recombined signal is measured. The Green, et al. paper does not suggest how to construct or operate reflectors which can be switched between an active state, in which a portion of light passing through the reflector is reflected, and an inactive state in which light passes through the sensor substantially unhindered. The Green, et al. detection scheme is a homodyne detection method employing feedback to a phase modulator to maintain, at the quadrature point, the phase of the two signals to be recombined. Green, et al. does not suggest any heterodyne detection technique, such as one in which the reflectors associated with a sensor themselves produce the optical frequency shift needed to facilitate heterodyne detection.